Orthogonal Frequency Division Multiple Access (OFDMA) techniques are an important part of multiple broadband access technologies. Long Term Evolution, the standard for fourth generation mobile communications, employs single carrier frequency division multiple access (SC-FDMA) in the uplink, a novel single carrier multiple access technique with similar performance and essentially the same overall structure as those of an OFDMA system, but with a better peak-to-amplitude power ratio. When combined with MU-MIMO techniques, SC-FDMA is able to deliver very high data rates with very good spectral efficiency and moderate complexity.
While OFDMA facilitates channel estimation and equalization tasks, the stringent requirements of time and frequency synchronization become an important issue. Time synchronization can be highly simplified through the insertion of a cyclic prefix (CP). Frequency offset between the transmitter and the receiver destroy the orthogonality among subcarriers, thus producing inter-carrier interference (ICI) that can strongly degrade its performance and has no straightforward solution.
A number of techniques have been proposed or suggested for the estimation and correction of frequency offset in the uplink. For example, Michele Morelli et al., “Synchronization Techniques for Orthogonal Frequency Division Multiple Access (OFDMA): A Tutorial Review,” Proc. of IEEE, Vol. 95, No. 7 (July 2007), describes iterative algorithms based on time domain space-alternating projection expectation-maximization (TD-SAGE). While effective, the disclosed time domain approach does not take advantage of the frequency separation of the different users, or the fact that most channel estimation methods are implemented in the frequency domain.
There are two aspects that significantly complicate the task of estimating and correcting frequency offset in the uplink. First, the carrier assignment scheme (CAS) plays in important role in maximizing the capacity of the channel, since the flexibility to freely distribute the available subcarriers between the different users based on the different channel conditions affecting each user, provide a form of “multi-user diversity.” A Generalized CAS allows a near unconstrained carrier assignment procedure that provides the maximum diversity gain but also complicates frequency offset estimation. In addition, in the uplink, each mobile user accessing the base station will be affected by a different amount of frequency offset.
A need therefore exists for methods and apparatus for simultaneous estimation of frequency offset and channel response for MU-MIMO OFDMA communication systems.